The muscle soreness felt a day or two after unfamiliar exercise is abbreviated as DOMS, short for delayed onset muscle soreness. It typically causes aching, tenderness, stiffness, and temporary weakness in muscles that have been worked in a new or unusually demanding way.
DOMS is especially associated with eccentric exercise, in which a muscle lengthens while producing force—for example, lowering a weight, running downhill, or descending stairs. The soreness usually becomes noticeable within 12 to 24 hours, peaks between 24 and 72 hours, and gradually fades over several days.
A common mix-up is the belief that DOMS is caused by lactic acid trapped in the muscles. Lactate levels generally return toward normal within about an hour after exercise, long before delayed soreness appears. The mechanism is more complex, but microscopic disruption and inflammatory responses in exercised muscle are thought to play important roles. The phenomenon was described in scientific work by Theodore Hough in 1902.